236 MISCELLANEOUS OBJECTIONS TO THJB 



tinned use, together with inheritance. With respect to 

 the mammae of tlie higher animals, the most probable con- 

 jcctui-e is that primordially the cutaneous glands over 

 the whole surface of a marsupial sack secreted a nutritious 

 fluid; and that these glands were improved in function 

 through natural selection, and concentrated into a confined 

 area, Tn which case they would have formed a mamma. 

 There is no more difficulty in understanding how the 

 branched spines of some ancient Echinoderm, which 

 served as a defence, became developed through natural 

 selection into tridactyle pedicellarice, than in understand- 

 ing tlie development of the pincers of crustaceans, through 

 slight, serviceable modifications in the ultimate and pe- 

 nultimate segments of a limb, which was at first used solely 

 for locomotion. In the avicularia and vibracula of the 

 Polyzoa we have organs widely different in appearance 

 developed from the same source; and with the vibracula 

 we can understand how the successive gradations might 

 have been of service. With the pollinia of orchids, the 

 threads which originally served to tie together the pollen- 

 grains, can be traced cohering into caudicles; and the steps 

 can likewise be followed by which viscid matter, such as 

 that secreted by the stigmas of ordinary flowers, and still 

 subserving nearly but not quite the same purpose, became 

 attached to the free ends of the caudicles — all these grada- 

 tions being of manifest benefit to the plants in question. 

 With respect to climbing plants, I need not repeat what 

 has been so lately said. 



It has often been asked, if natural selection be so potent, 

 why has not this or that structure been gained by certain 

 species, to which it would apparently have been advan- 

 tageous? But it is unreasonable to expect a precise answer 

 to such questions, considering our ignorance of the past 

 history of each species, and of the conditions which at the 

 present day determine its numbers and range. In most 

 cases only general reasons, but in some few cases special 

 reasons, can be assigned. Thus to adapt a species to new 

 habits of life, many co-ordinated modifications are almost 

 indispensable, and it may often have happened that the 

 requisite parts did not vary in the right manner or to the 

 right degree. Many species must have been prevented 

 from increasing in numbers through destructive agencies, 



